CA1281337C - Copolymer of difluoromethylene oxide and tetrafluoroethyleneoxide - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Perfluoropolyethers containing an approximate equal number of difluoromethylene and tetrafluoro-ethylene repeat units distributed either randomly or in alternating sequence along the polymer chain are disclosed. The polymers range in molecular weight from 500-200,000 amu; the lower molecular polymers being fluid and the intermediate and high molecular weight polymers being solid. The perfluorinated copolymers are synthesized by direct fluorination of corresponding methylene oxide/ethylene oxide copoly-mers.

Description

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COPOLYMER OF DIFLUOROMETH~LENE
OXIDE AND_TETRAFLUOROETHYLENE OXIDE

Field of the Invention This invention is in the field of fluorine 05 chemistry and more particularly in the field of direct fluorination.

Backyround PerfluoroalXylpolyethers are of current inter-est for many new material applications due to their lack of chemical reactivity and their outstanding thermal stability. Their remarkable stability, comparable to that of perfluoroalkanes, along with their intexesting surface properties, viscosities and broad liquid ranges make saturated perfluoro-polyethers attractive solvents, hydraulic fluids, heat transfer fluids, vacuum pump oils, lubricants, and grease base stocks. Yery high molecular weight perfluoropolyether solids have potential uses as sealants, elastomers, and plastics. See Paciorek, K.J.L, Kaufman, J., Nakahara, A., Journal of Fluo-rine Chemistry, 10, 277 (1977); McGrew, F.C., Chemical Engineering News, 45, 18 (August 7, 1967);
Eleuterio, H.~., Journal of Macromolecular Science-Chemistry, A6, 1027 (1972).
Several synthetic methods exist for preparing saturated perfluoropolyethers. ~he anionic poly~eri-zation of perfluoroepoxides, particularly hexafluoro-propylene oxide and tetrafluoroethylene oxide, have been used with success. See Hill, J.T., Journal of ~2~3~

Macromolecular Science-Chemistry, A6, 1027 (1972).
The preparation of perfluoropolyethers via this method first involves the oxidation of a perfluoro-olefin to a perfluoroepoxide, followed by an ionic 05 polymerization of the epoxide to an acyl fluoride terminated perfluoropolyether and conversion of the acyl fluoride end-groups to unreactive end-groups by decarboxylation reactions. The inability to form very high molecular weight pol,ymers, the lack of' stability of many perfluoroepoxides, and the e~treme di~ficulty encountered when attempting to polymerize substituted perfluoroepoxide have been cited as drawbacks associated with this art. Additionally, anionic polymerization of perfluoroepoxide does not lend itself well to the manufacturing of perfluoro copolymers since perfluoroepoxide vary widely in reactivity.
An alternative synthetic method for the pro-duction of perfluoropolyethers in~olves the UV
photolysis of tetrafluoroethylene and/or hexafluoro-propylene in an inert solvent in the presence of oxygen. This multistep process yields an acyl fluoride terminated polymer containing both the -CF2-, -CF2-CF2 CF~-CF2-, (CF2-CF2-O), and ~CF~CF3)-CF2-O) repeating units as well as unstable peroxidic oxygen linkages (CF2-O-O-CF2). Treatment of the polymer at elevated temperatures and with fluorine gas gives a stable polymer containing perfluoroalkyl ends groups. See U.S. Patents Nos. 3,665,041;
3,847,978; 3,770,792; and 3,715,378.

L33~7 Although this process can be used successfully to prepare copolymers, the process is completely random with little control of the kinds and numbers of repeating units. Undesirable linkages such as 05 the peroxidic oxygen and the poly~difluoromethylene) portions of the polymer are unavoidable and give the polymer undesirable properties for many applica-tions. The formation of by-product polytetrafluoro-ethylene and the need for fairly exotic solvents adds significantly to the production costs of the polymer.

Disclosure of the Invention This invention comprises substantially 1:1 random and l:1 altexnating copolymers of difluoro-methylene oxide and tetrafluoroethylene oxide. Theperfluoroethers are formed by controlled direct perfluorination of methylene oxidelethylene oxide copolymers.
Starting copolymers can be synthesized by ring-opening polymerization of 1,3-dioxolane.
1,3-dioxolane can be polymerized to give a copolymer of methylene oxide and ethylene oxide. Strict head-to-tail polymerization gives a 1:1 alternating - copolymer while random head-to-taili head-to-head polymerization gives a 1:1 random copolymer as depicted below:
. .

~2~3~33~
4-- .

H(-OCH2OCH2CH2)nOH Alternating H(-OCH2OCH2CH2)n~~(OCH2CH~OCH2)mOH Random When treated with elemental fluorine in a controlled manner, the following perfluorocarbon 05 polymers are formed:

Alternating copolymer - > X(-OCF2OCF2CF2)nOy Random copolymer ~ ( 2 2 2)n ( 2 2 F2)m wherein X and Y may be the same or different and are -CF3, -C2F5, -COF, -CF2OCF3, -CF2COF, -COOH, or -CF2COOH and n and m are integers greater than 1.
The molecular weight of the perfluoropolyethers can range from about 500 to about 200,000amu; the lower molecular weight polymers are fluids; the higher molecular weight polymers are solids.
The perfluoropolyether fluids of this invention are useful as hydraulic fluids, heat transfer media or as bases for high performance gr~ases which require fluids having a wide liquid range. The perfluoropolyether solids are useful as moldable elastomers or grease fillers. In addition, the solid polymers can be broken down, for example by pyrolysis at 600C, to produce low molecular weight fluids.

'.
' ~83L337 Best Mode of Carrying Out the Invention The difluoromethylene oxide/tetrafluoroethylene oxide polymers are produced by reacting elemental fluorine with a hydrocarbon polymer containing both 05 ethylene oxide and methylene oxide repeat units.
The preferred method of synthesizing the starting polymers is by polymerization of 1,3-dioxolane. The ring-opening polymerization of 1,3-dioxolane using a highly selective (i.e., sterospecific) catalyst such as Znsr2/triethylaluminum gives a strictly alterna-ting copolymer containing approximately equal numbers of ethylene oxide and methylene oxide repeating units. Polymers prepared from 1,3 dioxo-lane using less sterospecific catalysts such as strong acids can be used to prepare random copoly-mers. Polymers prepared by other synthetic tech-niques containing alternating or randomly distri-buted methylene oxide and ethylene oxide units along the polymer chain can be fluorinated to give a polymer similar to perfluoropolyethers prepared using polydioxolane.
The perfluoropolyethers of this in~ention are compounds, or mixtures thereof, having the following average formula:

~~(CF2CF2CF2)n~(CF2CF2CF2)m wherein X and Y are may be the same or different and are select from -CF3, -C2F5, -COF, -CF2OCF3, -CF2COF, -COOH, or -CF2COOH. Subscripts n and m are average indicia of composition such that when n and m are both greater than 1 and are approximately ~X~3~337 equal, a random copolymer is defined and when either n or m approaches zero in value, the polymer is referred to as an alternating copolymer which can be represented as follows:

05 X-(OCF2OCF2CF2)nOY

wherein X and Y may be the same or different and are -CF3, -C2F5, -COF, -CF2OCF3, -CF2COF, -COOH, or -CF2COOH and wherein n is an integer greater than 1.
Polymers containing intermediate values for n and m can be made, thus giving rise to properties common to both the random and alternating structures.
Because of the reacti~e nature of elemental fluorine, the ~aMar process is the preferred fluori-nation ';echnique. See R. J. Lagow and J. L. Mar-15 grave Progress in Inorganic Chemistry, 26, 161 (1979). When using such techniques, low concen-trations and small quantities of fluoxine are introduced initially in the fluorination reactor.
Typically, fluorine gas is diluted with nitrogen;
however, other diluents such as helium work equally as well. As the fluorination proceeds, higher fluorine concentrations and greater flows can be u'ilized without significant fragmentation of the pol~mer. Due to the extreme exothermic nature of the reaction, the fluorination ~ust be carried out slowly unless provisions have been made for removing the heat of reaction. Submersion of the reactor in a cooled liquid bath or the use of an internal FR~ON
cooling coil can satisfactorily removç the heat.

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' 3~7 Fluorine gas is the preferred fluorinating agent and is available commercially at sufficient purity levels. Other fluorinating agents such as chlorine trifluoride or bromine trifluoride can be Q5 used; however, some chlorine-or bromine substitution on the polymer generally will take place when these agents are used. The physical form of the polymer fluorinated is not critical; however, the fluorina-tion of fine powders work especially well.
The fluorination can be carried out by passing dilute fluorine over the polymer in a stationary reactor, in a rotating drum reactor, in a fluidized bed reactor or in a solvent reactor. The polymer may be soluble in the solvent (which must be inert to fluorine gas) or it may be present as a slurry.
Although a powdered polymer can be fluorinated in the neat form or in a solvent, the method of choice is to fluorinate the polymer in the presence of a hydrogen scavenger such as sodium fluoride (NaF) to adsorb the by-product hydrogen fluorideO The fluorination of ethers in the presence of hydrogen fluoride scavengers is described in Canadian Patent Application Serial Number 522~462, filed November 7, 1986 entitled "Perfluorination of Ethers in the Presence of Hydrogen Fluoride Scavengers"
- A 5:1 ratio of NaF to polymer is preferred, however, a 4:1 ratio also works well. Higher concen-trations of NaF do not show a significant additional positive effect.
The La~ar direct fluorination of a polyether containing both ethylene oxide and methylene oxide units can be illustrated as follows:

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- -:

1,2~3~33~7 F2/He -(CH2CH2-0-CH2-0)- ~ X-(CF~CF2-0-CF2)n~Y
1. T=amb 2. ~

wherein X and Y may be the same or different and are defined as -CF3, -C2F5~ -COF, -CF2OCF3, -CF2COF, COOH, or CF2COOH and n is an integer greater than 1.
05 Perfluoroethers of a broad range of molecular weights ~500-200,000 amu) can be prepared depending upon the molecular weight of the starting hydro-carbon material and the fluorination conditions used. High fluorine concentrations, fast flow rates and elevated temperatures each favor fragmentation, thus lower molecular weight products are obtained.
Milder fluorination conditions designed to prevent fragmentation lead to an extremely stable high molecular weight perfluoropolyether.
When mild fluorination conditions are used to fluorinate a high molecular weight polymer (greater than 20,000 amu), a white solid is typically obtained.
Several schemes can be employed to prepare interme-diate molecular weight fluids~ One scheme is to perfluorinate a low molecular weight polymer using mild fluorination conditions. Treating a higher molecu7ar weight polymer with slightly harsher fluorination conditions can lead to fluids when the conditions are chosen to give a controlled amount of ~L21!3~33~7 g chain cleavage. "Perfluorination" of a high mole-cular weight polymer using mild conditions can be used to replace a specified number of hydrogens with fluorine. A second step is designed to promote 05 fragmentation. ~levated temperatures and high fluorine concentrations are used to give the per-fluoropolyether fluid.
An alternate scheme J and possibly the method of choice for preparing a wide range of molecular weights involves the fluorination of a high mole-~ular weight polymer using mild fluorination condi-tions to give a high molecular weight solid contain-ing both the perfluoro alkyl and acyl fluoride end groups. Treatment of the polymer with pure fluorine at elevated temperature ( 100C) gives a polymer containing only perfluoro alkyl end groups. The resulting high molecular weight solids can be broken down to lower molecular wei-ght components by pyrolysis.
This procedure is described in Canadian Patent Application Serial No. 522,463, filed November 7, 1986 entitled "Pyrolysis of Perfluoropolyethers";
Pyrolysis of the solid in thepresence of nitrogen, air or fluorine gives lower molecular weight polvmers. By selecting the proper pyrolysis temperature ~400-500C) and by carrying out the pyrolysis in a distillation-tvpe apparatus, a well-defined boi ing point range can be collected while less volatile components are returned to the high temperature portion of the apparatus to be further fragmented. If the -c ~

~2~L33~7 pyrolysis is not carried out in the presence of fluorine r an additional fluorination at elevated temperatures is needed to remo~e the acyl fluoride terminal groups.
05 Various terminal groups are obtained in the fluorination and pyrolysis reactions. For many applications where an inert material is required, it is desirable to remove acid and acyl fluoride end groups. This is best accomplished by treating the polymer with pure F2 at a temperature greater than 100C. Some of the reactions occurring are re-presented by the following equations where Pf corresponds to a perfluorinated polyether chain.

Pf-O-CF2-COF ~ f O CF3 + COF2 Pf-CF2-O-CF2-O-COF ~ pf-CF3 + 2COF2 + 1/~ 2 Pf-O-CF2-CF2-O-COF ~ pf-o-cF2-cF3 + COF2 + 1/2 2 f 2 ~ Pf O-CF3 + CO2 + HF

In addition to reactions of this type which relate exclusively to the terminal groups of the polymer, fluorine can react at elevated temperatures with stray hydrogens left on the polymer resulting 3~'7 in chain cleavage at that point. However, at 100C, approximately 80% of the remaining hydrogens can be replaced with fluorine without chain degradation providing that fewer than 1~ of the hydrogens remain 05 in the polymer. Typically, upon completing the fluorination at elevated temperatures, the hydrogen content of the polymer is below 5ppm as determined by Fourier transform infrared spectroscopy.
The perfluoropolyether fluids of this invention have distinct advantages over the existing fluid, namely Fomblin Z fluids. Fomblin Z fluids have a widely varying structure containing repeating units such as polydifluoromethylene, -CF2CF2CF2- and -CF2CF2CF2CF2- which can increase the viscosity of the fluid at low temperatures. F NMR analysis of Fomblin zTM fluids shows that the fluid structure is less random than previously thought and that the ethylene oxide and methylene oxide units tend to be present in blocks. Three or more sequential methy-lene oxide units act as a weak point in the polymerchain and limit the thermal and oxidative stability of Fomblin Z fluids. Perfluoropolyethers of this invention contain either 1 or 2 methylene oxides in a row depending upon the starting material used. Like Fomblin zTM fluids, the polymers contain difluoromethylene oxide units (for good low tempera ture properties) and tetrafluoroethylene oxide (for improved high temperature stability).
The invention is illustrated further by the following examples:

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Example 1 1,3~dioxolane was polymerized by placing 250g of the dried material in a nitrogen-purged l L
flask. 1.6g of ZnBr and 3.5 cc of a 53 triethyl-05 aluminum in toluene solution was added to the flask.
After 3 days the polymerization was complete. The solid polymer was ground to 50 mesh or smaller using liquid nitrogen in a blender.
2g of the sieved polydioxolane powder were mixed with lOg of 100 mesh NaF powder in a nickel boat which was placed in an 18n long reactor con-structed from 1 1/2" nickel pipe containingTEFLON
O-ring sealed flanged ends. The assembled apparatus was flushed with lOOcc/min of N2 for several hours before beginning the fluorination. The nitrogen flow was monitored with a glass rotameter while the fluorination flow rate was controlled with a Monel needle val~e and monitored with a Hastinss mass flow transducer, Type F-SOM. The fluorine, supplied by Air Products, was used without further purification~
The fluorine flow was set at 2cc/min while the N2 flow was maintained at lOOcc/min for 2 days. After ~8 hours of relatively mild conditions, pure fluo-rine was used for 5 hours followed by 5 hours of exposure to pure fluorine at 110C to remove any acyl fluoride terminal groups. Upon completing the fluorination at elevated temperatures, the apparatus - was again flushed with lOOcc/min N2 fo~ approxi-mately one hour.
The solid reaction product was stirred with 75ml of FREON 113 for approximately 1 hour. Upon * Trade mark ~Z8~L33~

removing the solid by filtration, l.9g of a low volatility, low viscosity oil was recovered from the F~EON. The oil, when placed in a freezer held at -50C, continued to flow well.
05 The FREON insoluble portion was washed with approximately 300cc of distilled water to dissolve away the NaHF~ leaving behind 0,8g of a white free flowing powder which is a higher molecular weight version of the oil obtained (Tol:al yield: 54~9~).
The fluid was characterized by 9F NMR. Each of the individual spectral lines were assigned to a structure by comparison with the spectra of known perfluoro compounds. Spectral data ~or the fluid is summarized in the table below:

Table ChemicalRelative Structure Shift (ppm) IntensitY (%) -CF3cF2cF2O-- SO.O 1.9 -OCF2CF2OCF2OCF2CF2O- 53.224.0 CF3OCF2-0- 5S.5 3.6 CF3OCF2CF20- 57.3 5.2 CF3OCF20- 59.2 4.5 CF3CF20- ~ 89.0 2.9 CF3C-2 1.9 OCF2OCF2CF20- 92.551.3 On the basis of the NMR spectroscopic analysis, the average structure was the fluorocarbon analogue -of the hydrocarbon starting material polydioxolane.

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~2~33t7 Example 2 300g of polydioxolane powder was dissolved in 500ml of methylene chloride and mixed with 1200g NaF
powder. The solvent was evaporated and the resul-05 ting solid was ground cryogenically to give a powderwhich will pass a 50 mesh screen. The powder was placed in a 9" ID x 2' long aluminum drum reactor which rotates at 5 rev./min/ The reactor was flushed with nitrogen for several hours prior to beginning the fluorination. A gas flow of 300 cc/min fluorine and 2 L/min nitrogen was maintained for 36 hours. The nitrogen was decreased to 1 L/min for an additional 12 hours. The polymer is treated with-pure fluorine for several hours to insure perfluorination. A reactor temperature between OC
and +20C was desirable for best results. A final fluorination at 110C for 4 hours was used to replace any residual hydrogen with fluorine and to convert reactive acyl fluoride end groups to inert trifluoromethyl or pentafluoroethyl terminal groups.
Extraction of the powder with 2 liters of FREON 113 gave 370g of the desired diflucromethylene oxide-tetrafluoroethylene oxide copolymer. An additional 160g of a FREON lnsoluble solid was also obtained which Gan be converted to a fluid ~ia pyrolysis.
Elemental analysis for solid: calculated (C3F6O2,~:
C, 19.80; F, 62.63, found: C, 18.11; F, 62.53.

Example 3 Two grams of polydioxolane were placed in a nickel boat along with lOg of NaF pellets (1~8"
mesh). The boat was placed in a 1 1/~ nickel tube reactor and flushed with lOOcc/min N2 prior to beginning the fluorination. The fluorine and , , 3~

nitrogen flow rates were set at 2cc/min and 100cc/min, respectively. After 48 hours had elapsed, the sample was treated for 12 hours with pure fluorine at 100C. Extraction of the product mixture with 05 FREOM 113 ga~e 1.5g of a clear, low viscosity, nonvolatile oil. The NaF/NaHF2 pellets were screened from the sample leaving behind 0.~g o~ a white solid (Total yield: 38.6%). Infrarecl analysis and the ~MR spectra of the oil were very similar to that observed for the oil prepared according to Example 1.

Example 4 Fluorination of polydioxolane using the very mild conditions as described in Examples 1 and 2 gives a perfluoro product with a minimal amount of chain degradation occurring during the fluorination reaction. The oil present in the sample results from the direct fluorination of lower molecular weight chains in the hydrocarbon starting material.
The oil to solid ratio of the final product can be increased by employing a two-step direct fluorina-tion process. In the initial phase, dilute fluorine is passed over the sample to replace the majority of the hydrogen. The second step, perfluo-ination of the sample with pure fluorine at elevated tempera-ture, give a product with a lower average molecular weigsht. The exothermi5ity of the reaction with elemental fluorine ~esults in some chain fragmen-tation.
Two grams of polydioxolane was mixed with 10g of NaF powder. The reactor was purged with L33~7 lOOcc/min N2 for 1 hour, followed by reaction of the polymer with 2cc/min F2 diluted with lOOcc/min N2 for 48 hours. Next, the polymer was subjected to pure fluorine at 100C for 8 hours at which time 05 some chain cleavage occurred. Using this procedure, 2.4g of oil and O.lg of solid material are obtained (50.8~ total yield).

Industrial Applicability The difluoromethylene oxide/tetrafluoroethylene oxide fluids of this invention are useful as oils, hydraulic fluids or as bases for high performance greases which require fluids having a wide liquid range. The fluids can be prepared in the molecular weight range desirable for a particular use. For example for vacuum pump oils, fluids ranging in molecular weight from about 5,000 to about 20,000 amu are desirable. Fluids ranging from about 750-2,000 amu are useful as vapor phase soldering fluids and those ranging from about 1,000-3,000 as hydraulic fluids. The perfluoropolyether solids are useful as moldable elastomers or grease fillers.
In addition, the solid polymers can be broken down, for example by pyrolysis, at 500-600C to produce low molecular weight fluids.
The perfluormethylene oxide/ethyl~ne oxide -polymers of this invention have both very good thermal stability and excellent low temperature properties. They are devoid of particular molecular structures believed to be associated with poor thermal stability and high fluid viscosity.

Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine experi-mentation, many equivalents to the specific embodi-ments o~ the invention described herein~ Such equivalents are intended to be encompassed by the following claims.

Claims (8)

1. Perfluoropolyethers of the formula:

X-(OCF2OCF2CF2)nOY
wherein X and Y may be the same or different and are -CF3, -C2F5, -COF, CF2OCF3, -CF2COF, -COOH, or -CF2COOH and wherein n is an integer greater than 1;
the perfluoropolyethers having a molecular weight of about 500 to about 200,000 amu.

2. Perfluoropolyether fluids of the formula:
X(OCF2OCF2CF2)nOY

wherein X and Y may be the same or different and are -CF3 or -C2F5 and wherein n is an integer greater than 1 such that the fluids range in molecular weight from 750-20,000 amu.

3. Perfluoropolyethers having an average formula of X-(OCF2OCF2CF2)nOY wherein the terminal groups X and Y are selected from the group consisting of -CF3, -C2F5, -COF, -CF2OCF3, -CF2COF, -COOH and -CF2COOH and wherein n is an integer greater than 1;
the perfluoropolyethers having a molecular weight of about 500 to about 200,000 amu.

4. Perfluoropolyethers according to claim 3, wherein the terminal groups X and Y are -CF3 or -C2F5.

5. A method of preparing perfluoropolyethers having an average formula of X-(OCF2OCF2CF2)nOY
wherein the terminal groups X and Y are selected from the group consisting of -CF3, -C2F5, -COF, -CF2OCF3, -CF2COF, -COOH and -CF2COOH and wherein n is an integer greater than 1, the perfluoropolyethers having a molecular weight of about 500 to about 200,000 amu, comprising the steps of:

a. providing a copolyether consisting essen-tially of methylene oxyde and ethylene oxide units in a molar ration of about 1:1; and b. perfluorinating the copolyether by reacting the copolymer with elemental fluorine under controlled conditions such as to produce a perfluoropolyether.

6. A method according to claim 5, wherein the perfluorination step (b) is carried out by:

i) exposing the copolymer to a mixture of an inert gas and fluorine gas, the fluorine concentration being from about 1 to about 10%; and ii) increasing the concentration of fluorine gas until the polymer is exposed to pure fluorine gas thereby perfluorinating the copolyether to produce a perfluoropolyether.

7. A method according to claim 5 or 6, wherein the fluorination is accomplished in the presence of sodium fluoride.

8. A method according to claim 6, further com-prising:

iii) treating the polymer with fluorine gas at an elevated temperature greater than 100°C sufficient to convert any acid or acyl fluorine end groups to per-fluoroalkyl groups.